A 160 ksi Grade Economy Non-Standard Oil Casing and Its Manufacturing Method

Abstract

The present invention relates to a 160 ksi grade economical non-standard oil casing and a manufacturing method thereof. The chemical composition by weight percentage includes: C 0.10% - 0.40%, Si 0.05% - 0.35%, Mn 0.55% - 2.50%, Cr 0.50% - 2.50%, Mo 0.10% - 0.50%, V 0.05% - 0.25%, Nb ≤ 0.05%, B 0.0005% - 0.002%, Ti 0.01% - 0.05%, Al ≤ 0.05%, P ≤ 0.015%, S ≤ 0.005%, and the balance is Fe and inevitable impurity elements. The advantages are as follows: The combination of hot rolling and direct online quenching can eliminate the need for secondary heating, save energy, and improve production efficiency; by controlling the quenching end temperature to meet the requirements of the tempering holding temperature, the energy consumption of the tempering process can be reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of manufacturing ultra-high strength structural steel, and particularly relates to an economical non-standard oil casing pipe with a yield strength not less than 160 ksi and a manufacturing method thereof. Background Art

[0002] Seamless pipes with a yield strength not less than 160 ksi can meet the material selection requirements in the field of high-strength seamless pipes that allow quenched and tempered state supply, and are suitable for producing seamless pipe products such as tubing, casing pipes, and couplings for deep wells and ultra-deep wells.

[0003] Oil well pipes are one of the necessary equipment in oil and gas development projects and are a special material. Generally speaking, 20%-30% of the cost of an oil and gas well is the cost of oil well pipes. In the total amount of steel used in the petroleum industry, oil well pipes can account for 40%, and casing pipes account for more than 90% of oil well pipes.

[0004] Currently, easily exploitable shallow oil and gas resources are gradually exhausted, prompting the depth of oil and gas wells to gradually increase and the well depth growth rate to accelerate year by year. After the well depth increases, the pressure and temperature in the well continuously increase, the geological environment becomes more severe, and the stress state of the casing pipe is more complex and severe. The highest grade Q125 steel grade of existing API casing pipes can no longer meet the requirements of oil and gas development transferred to the western region and the ocean. The safe operation of deep wells and ultra-deep wells urgently requires high-toughness steel for oil and gas development above 160 ksi.

[0005] With the increase in strength, there are various problems with the composition or preparation process of existing seamless pipes with a strength exceeding Q125 steel grade. Although the yield strength of some product samples can reach the 160 ksi requirement, these technical solutions still have disadvantages such as a high content of added alloying elements, a complex production process, and poor strength-toughness matching.

[0006] The patent application number is CN201811165255.8, which discloses a seamless steel pipe of 140ksi grade and its manufacturing method. The yield strength of this steel pipe is 980 - 1120MPa, and the full-size Charpy impact energy at 0℃ is not less than 120J. In terms of composition design, 0.0005% - 0.0040% of B element and 1.00% - 3.00% of Ni element are added. In the production process, the tube blank is produced by centrifugal casting to obtain a hollow tube blank, and then rolled into the specified size by an MPM / PQF continuous rolling mill and heat-treated. The patent application number is CN200910069758.X, which discloses a steel pipe for downhole operations of 150ksi grade with high strength and toughness and its production method. The yield strength of this steel pipe is 1034 - 1148MPa, the transverse half-size Charpy impact energy at 0℃ is 46 - 53J, and the longitudinal 3 / 4-size Charpy impact energy is not less than 87 - 101J. In terms of composition design, B element is not added, and 0.70% - 0.80% of Mo element is added. In the production process, feeding Si-Ca wire is used to change the morphology of inclusions, essentially improving the toughness and low-temperature toughness of the steel. The patent application number is CN201310409266.7, which discloses an oil casing for ultra-deep wells and its production process. The yield strength of this steel pipe is 1158 - 1167MPa. In terms of composition design, 0.02% - 0.05% of Ce element is added. The patent application number is CN202111540297.7, which discloses a high-strength Nb-containing oil casing based on controlled cooling and its manufacturing method. The yield strength of this steel pipe is 1180 - 1300MPa, and the transverse impact energy at 0℃ is 80 - 150J. In terms of composition design, Ni is added, and B is selectively added; in the production process, the process methods of "continuous cooling" and "sparse cooling" are adopted, with an average cooling rate of 20 - 60℃ / s to obtain a duplex structure mainly composed of bainite and with the content of ferrite + pearlite not exceeding 10%, and then tempered sorbite structure is obtained after heat treatment to ensure the requirement of strength and toughness matching. Summary of the Invention

[0007] The object of the present invention is to provide a 160ksi grade economical non-standard oil casing and its manufacturing method. By appropriately increasing the contents of Mn and Cr and strictly controlling the contents of Mo and B elements, a 160ksi grade seamless pipe is produced using a traditional solid round billet to obtain an ultra-high strength seamless pipe with a yield strength not less than 160ksi and excellent low-temperature toughness.

[0008] To achieve the above object, the present invention is realized through the following technical solutions:

[0009] A 160ksi grade economical non-standard oil casing, the chemical composition by weight percentage includes:

[0010] C 0.10% to 0.40%, Si 0.05% to 0.35%, Mn 0.55% to 2.50%, Cr 0.50% to 2.50%, Mo 0.10% to 0.50%, V 0.05% to 0.25%, Nb ≤ 0.05%, B 0.0005% to 0.002%, Ti 0.01% to 0.05%, Al ≤ 0.05%, P ≤ 0.015%, S ≤ 0.005%, the balance being Fe and unavoidable impurity elements.

[0011] In this casing pipe, the size tp of the C precipitation phase containing Mo and V does not exceed 200 nm, the size tp of the C precipitation phase containing Mo and V with a volume fraction of more than 75% does not exceed 50 nm, the transverse Charpy impact Akv at 0 °C is not less than 120 J, and the longitudinal Charpy impact Akv is not less than 150 J.

[0012] A manufacturing method for a 160 ksi grade economical non-standard casing pipe, comprising the following steps:

[0013] 1) Steel billet smelting and continuous casting: Molten iron is first smelted in an electric furnace, fed into an LF furnace for refining, and subjected to vacuum treatment in a VD furnace before being continuously cast into a round pipe billet;

[0014] 2) Steel pipe rolling: The round pipe billet is heated to 1205 - 1255 °C, the temperature before piercing is 1160 - 1220 °C, the temperature before skew rolling is 950 - 1130 °C, and the temperature before stretch reducing is 850 - 920 °C;

[0015] 3) Quenching and tempering treatment: After stretch reducing is completed, the steel pipe is directly quenched online. The starting temperature of quenching is 830 - 870 °C, the ending temperature of quenching is 200 - 400 °C. After quenching, the steel pipe is directly tempered while still hot, the tempering temperature is 200 - 400 °C, and it is air-cooled after tempering.

[0016] In step 3), the quenching medium is water or oil, and the average cooling rate of the quenching medium is above 65 °C / s.

[0017] The holding time t T (unit: minute) of the tempering in step 3) satisfies:

[0018] t T =(4 - 6)t (1)

[0019] In formula (1), t T is the holding time of tempering, with the unit of min; t is the wall thickness of the steel pipe, with the unit of mm.

[0020] The functions and ranges of the main alloying elements of the 160 ksi grade economical non-standard casing pipe are described as follows:

[0021] Carbon C: C is the main element in steel second only to Fe, which directly affects the properties of steel such as strength, plasticity, and toughness. C has an obvious effect on improving the strength of steel through solid solution strengthening and precipitation strengthening, but increasing the C content has a negative impact on the plasticity and toughness of steel. Therefore, the C content range is set to 0.10% - 0.40%.

[0022] Silicon Si: Si is an important reducing agent and deoxidizer in the steelmaking process. It can dissolve in ferrite and austenite to increase the hardness and strength of steel. Increasing the Si content can reduce the precipitation tendency of Fe3C. If the Si content is too high, the plasticity and toughness of steel will be significantly reduced. Therefore, the Si content range is set to 0.05% - 0.35%.

[0023] Manganese Mn: Mn can improve the hardenability of steel, which is beneficial to the strength of steel; it can eliminate the influence of S (sulfur) and improve the hot working performance of steel. Since Mn is relatively cheap and can be infinitely solid-solved with Fe, while increasing the strength of steel, its influence on plasticity is relatively small. Therefore, Mn is widely used as a strengthening element in steel. If the Mn content is too high, it will exacerbate the segregation of continuous casting billets, increase the grade of strip structure in steel pipes, and the tissue uniformity becomes worse, which is not conducive to the plasticity and low-temperature toughness of steel pipes. Therefore, the Mn content range is set to 0.55% - 2.50%.

[0024] Chromium Cr: Cr can increase the hardenability of steel and has a secondary hardening effect. It can improve the strength, hardness, and wear resistance of steel without making the steel brittle, but it will reduce the elongation and reduction of area. The main role of Cr in the quenched and tempered structure is to improve the hardenability, so that the steel has better comprehensive mechanical properties after quenching and tempering. If too much Cr is added, during the tempering process, Cr-containing carbides precipitate and aggregate and grow at the original austenite grain boundaries, seriously damaging the low-temperature toughness of steel pipes. Therefore, the selected Cr content range is 0.50% - 2.50%.

[0025] Molybdenum Mo: The role of Mo is similar to that of Cr. Due to its high price, the addition amount should not be too high. Therefore, the selected Mo content range is 0.10% - 0.50%.

[0026] Vanadium V: V has a very strong affinity with C, N, and O, and forms corresponding stable compounds with them. V mainly exists in the form of carbides in steel, which has the effects of refining the structure and grains, improving strength and toughness, and reducing overheating sensitivity. Vanadium can increase the tempering stability of quenched steel and produce a secondary hardening effect; in quenched and tempered steel, it mainly improves the strength of steel. Therefore, the selected V content range is 0.05% - 0.25%.

[0027] Titanium Ti: Ti has a strong affinity with C, N, and O, and forms corresponding stable compounds with them. It is one of the most important N-fixing elements. The precipitates containing Ti have strong binding force, are stable, and are not easy to decompose. They can prevent the grain growth tendency of steel at high temperatures and improve the welding performance of steel. Using Ti to fix N and S is beneficial to improving the strength and plasticity of steel. Increasing the Ti content will coarsen the Ti-containing precipitate phase and have an adverse effect on the performance. The core role of Ti in the present invention is to fix N and prevent N from combining with V to affect the coordinated precipitation of V and Mo. The present invention selects a Ti content range of 0.01% to 0.05%.

[0028] Niobium Nb: Nb is one of the most important microalloying elements. It partially dissolves into the solid solution and plays a role in solid solution strengthening. When it exists in the form of carbides, nitrides and oxide particles, it can increase the tempering stability of steel and has a secondary hardening effect. Trace amounts of Nb can increase the strength of steel without affecting the plasticity or toughness of the steel. Due to the effect of refining grains, it can improve the impact toughness of steel and reduce its brittle transition temperature. During the rolling process, solid-solution Nb significantly increases the recrystallization temperature of the steel, which can complete the rolling process of the steel within a higher temperature range, thereby reducing the internal stress of the steel pipe. The present invention selects a Nb content of no more than 0.05%.

[0029] Aluminum Al: Al is added to steel as a deoxidizer or alloying element. Aluminum's deoxidation ability is much stronger than silicon and manganese. The main role of aluminum in steel is to refine the grains and fix nitrogen in the steel, thereby significantly improving the impact toughness of the steel and reducing the cold brittleness tendency and aging tendency; aluminum can also improve the corrosion resistance of steel, especially when used in combination with molybdenum, copper, silicon, chromium and other elements, the effect is better; the disadvantage of aluminum is that it affects the hot working performance, welding performance and cutting performance of steel. The present invention selects the Al content range not higher than 0.05%.

[0030] Phosphorus P: P is brought into steel from ores and is one of the harmful elements like S. Although P can increase the strength and hardness of steel, it causes a significant decrease in plasticity and impact toughness. Especially at low temperatures, it makes steel significantly brittle. The higher the P content, the greater the cold brittleness. Removing P to a lower level will significantly increase the cost of steelmaking. The present invention selects a P content range of no more than 0.015%.

[0031] Sulfur S: Sulfur S comes from steelmaking ore and fuel coke. It is one of the most common harmful elements in steel and is detrimental to the ductility, toughness, weldability and corrosion resistance of steel. If S exists in the form of FeS in steel, it can also cause "hot brittleness" during hot working. The present invention selects the S content range to be no higher than 0.005%.

[0032] The reasons for the control range of various parameters in the manufacturing method of 160ksi economical non-standard oil casing are as follows:

[0033] The present invention adopts the composite precipitation strengthening of elements such as Cr, Mo, V, and Nb, controls the heating temperature of the continuous casting round tube billet at 1205 - 1255 °C, and the total time in the furnace is 2 - 4 h, ensuring that the precipitation phases of alloy elements are fully redissolved into austenite, and fully exerting the beneficial effects of inhibiting recrystallization, solution strengthening, precipitation strengthening, and grain refinement in the subsequent process, so as to make composition and temperature preparations for obtaining the final microstructure. Below the selected temperature and time range, the solution will be insufficient, affecting the strength of the final steel pipe; above the selected time and temperature range, the original austenite grains of the continuous casting billet are prone to be overly coarse, which is not conducive to the control of the toughness of the steel pipe.

[0034] After rolling the steel pipe, direct online quenching can be selected or offline reheating followed by quenching can also be chosen. By appropriately controlling the production process, it can be ensured that the temperature of the tube after reducing the diameter meets the quenching requirements, making it easy to achieve direct online quenching, saving energy, reducing production costs, reducing the reheating process, and improving production efficiency, with significant economic benefits.

[0035] After the accelerated cooling of the steel pipe is completed, tempering heat treatment is carried out. When the tempering holding temperature is higher than 400 °C, the strength of the steel pipe decreases significantly, which is not conducive to the final strength and toughness matching of the steel pipe; while when it is lower than 200 °C, the tempering of the quenched structure is insufficient and the low-temperature toughness is low. If the tempering holding time is too long, the strength deteriorates; if the tempering holding time is too short, the toughness is insufficient. By controlling the appropriate tempering temperature, tempering holding time, and the contents of key alloy elements Mo, V, and Ti, it is ensured that beneficial precipitation phases are fully precipitated and have small sizes. The size t of the precipitation phase (Mo, V)C p does not exceed 200 nm, and the size t of the precipitation phase (Mo, V)C with a volume fraction of more than 75% p 75％ does not exceed 50 nm. Thus, under a relatively wide quenching and tempering process window, good strength and toughness matching and good performance stability characteristics are achieved.

[0036] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0037] 1) During the steel pipe rolling process, there is no need to specially control the rolling temperature and deformation amount. As long as the quenching temperature is satisfied before tension reducing, the combination of high-temperature rolling and direct online quenching can eliminate the need for secondary heating, save energy, and improve production efficiency; by controlling the quenching end temperature to meet the requirements of the tempering holding temperature, the energy consumption of the tempering process is reduced.

[0038] 2) Mo is used instead of N element to promote the effective precipitation of V element. Compared with V(C, N), (Mo, V)C has higher thermal stability and a more spherical morphology, and can achieve more stable strength and toughness matching effects within a wider range of heat treatment process parameters.

[0039] 3) The yield strength Rt0.7 of the steel pipe produced is ≥ 1100 MPa, the tensile strength Rm ≥ 1200 MPa, the elongation A ≥ 15%, and the transverse and longitudinal Charpy impact energy Akv at 0 °C are not less than 120 J and 150 J respectively. Detailed implementation mode

[0040] The present invention will be described in detail below, but it should be noted that the implementation of the present invention is not limited to the following embodiments.

[0041] The chemical compositions of the 160 ksi grade economical non-standard oil casing examples are shown in Table 1; the rolling and heat treatment process parameters of the steel for the 160 ksi grade economical non-standard oil casing examples are shown in Table 2; the mechanical properties of the steel for the 160 ksi grade economical non-standard oil casing examples are shown in Table 3.

[0042] Table 1 Chemical compositions of the steel pipes in the examples of the present invention wt%

[0043] Example C Si Mn P S Cr Mo Nb V Ti Al B* 1 0.11 0.34 0.56 0.01 0.005 0.55 0.20 0 0.24 0.02 0.04 12 2 0.23 0.24 0.76 0.01 0.003 1.05 0.11 0.05 0.06 0.01 0.02 18 3 0.31 0.14 0.96 0.01 0.003 1.55 0.40 0.04 0.15 0.05 0 8 4 0.39 0.06 1.15 0.01 0.005 2.05 0.15 0.03 0.10 0.04 0.01 20 5 0.15 0.19 1.45 0.01 0.005 2.45 0.17 0.02 0.25 0.01 0.03 10 6 0.28 0.29 1.75 0.01 0.003 2.35 0.25 0.01 0.19 0.02 0.02 6 7 0.18 0.10 1.95 0.01 0.003 1.85 0.35 0.03 0.10 0.02 0.04 20 8 0.25 0.25 2.15 0.01 0.005 1.35 0.45 0 0.05 0.03 0.03 15 9 0.25 0.25 2.45 0.01 0.004 0.75 0.49 0.03 0.15 0.02 0.01 15

[0044] Note: * represents the content calculated in ppm.

[0045] Table 2 Rolling and heat treatment process parameters of the steel in the examples of the present invention

[0046]

[0047] Table 3 Mechanical properties of the steel in the examples of the present invention

[0048]

[0049] It can be seen from the data in Tables 1 to 3 that the steel pipes prepared by the method of the present invention have a yield strength ≥ 1110 MPa, a tensile strength ≥ 1200 MPa, an elongation ≥ 15%, and the transverse and longitudinal Charpy impact energies at -0 °C are not less than 120 and 150 J respectively, with excellent strength and toughness matching and low cost.

Claims

1. A 160 ksi grade economic non-standard oil casing pipe, characterized in that, The chemical composition by weight percentage includes: C 0.10% - 0.15%, Si 0.05% - 0.35%, Mn 1.15% - 2.50%, Cr 2.05% - 2.50%, Mo 0.10% - 0.50%, V 0.05% - 0.25%, Nb ≤ 0.05%, B 0.0005% - 0.002%, Ti 0.01% - 0.05%, Al ≤ 0.05%, P ≤ 0.015%, S ≤ 0.005%, and the balance is Fe and inevitable impurity elements; In this casing pipe, the size t of the C precipitation phase containing Mo and V p does not exceed 200 nm, and the size t of the C precipitation phase containing Mo and V with a volume fraction of more than 75% p 75％ does not exceed 50 nm, the transverse Charpy impact Akv at 0 °C is not less than 120 J, and the longitudinal Charpy impact Akv is not less than 150 J; Rt0.7 / Rm is 0.9 or 0.89; The manufacturing method of the 160 ksi grade economic non-standard oil casing includes the following steps: 1) Steel billet smelting and continuous casting: Molten iron is first smelted in an electric furnace, sent to an LF furnace for refining, and subjected to vacuum treatment in a VD furnace before being continuously cast into a round tube billet; 2) Steel pipe rolling: The round tube billet is heated to 1205 - 1255 °C, the temperature before piercing is 1160 - 1220 °C, the temperature before skew rolling is 950 - 1130 °C, and the temperature before stretch reducing is 850 - 920 °C; 3) Quenching and tempering treatment: After stretch reducing is completed, the steel pipe is directly quenched online. The starting temperature of quenching is 830 - 870 °C, the ending temperature of quenching is 200 - 400 °C. After quenching, the steel pipe is directly tempered while still hot, the tempering temperature is 200 - 400 °C, and it is air-cooled after tempering; The quenching medium is water or oil, and the average cooling rate of the quenching medium is above 65 °C / s; The soaking time t of the tempering T (unit: minute) satisfies: t T = (4 to 6)t (1) In formula (1), t T is the holding time of tempering, with the unit of min; t is the wall thickness of the steel pipe, with the unit of mm.

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